The present invention relates to an ultrasonic imaging system and an ultrasonic imaging method for generating images of a living body with ultrasonic waves.
An ultrasonic imaging system used for medical imaging diagnosis can display a tomographic image of a tissue of a soft part in a living body, an image of a blood flow in a living body, and the like in an almost real-time manner on a monitor by using the ultrasonic pulse echo method so that the images can be observed. Since a living body is not exposed to radiation which is used in an image diagnosing system, the ultrasonic imaging system is very safe. In addition, the system is small in size and cheap, so that it is used widely in the medical field.
An ultrasonic tomographic image (B-mode image) is an image indicative of the position of a reflector estimated from time required since ultrasonic waves are transmitted until an echo signal is received and intensity of the echo signal by transmitting ultrasonic waves to a living body and receiving an echo signal reflected from a region in the living body in which acoustic impedance changes spatially. It is known that peculiar artifact called speckle occurs in ultrasonic imaging. To improve the quality of an image, it is desirable to minimize the speckle.
Hitherto, a method and apparatus for adaptively enhancing a B-mode image has been proposed (for example, refer to Japanese Patent Application Laid-Open No. 11-197151). In the B-mode image enhancing apparatus, a low pass filter which smoothes out speckle and a high pass filter which enhances edges are placed in parallel signal paths connected to the output of an envelope detector. The signals in the high pass filter path are logarithmically compressed before high pass filtering. The signals in the low pass filter path are logarithmically compressed after low pass filtering. Respective weighting factors are applied to the low- and high-pass-filtered signals by an adaptive weighting means. The weighted low- and high-pass-filtered signals are summed and optionally input to an anti-aliasing filter before decimation and scan conversion.
A method of extracting a microstructure in an RF signal using statistical similarity has been proposed (for example, refer to Kamiyama et al., “Method for extracting micro-structure in RF signal using statistical similarity”, Papers of Basic Technical Research of The Japan Society of Ultrasonics in Medicine, Dec. 22, 2001, Vol. 101, No. 4, pp. 14-18). The method is characterized in that by a spatial filtering for assigning a weight according to “similarity” in which Rayleigh probability density is assumed on a reception signal in a sample, a σ value to be referred to is estimated, so that an influence of attenuation of a living body or the like can be avoided.
When the method was applied by using RF signals of a normal liver and a liver suffering cirrhosis, micro scatterers displaying non-Rayleigh scattering could be extracted while relatively maintaining drawing property of image diagnosis.
In the non-patent document 1 mentioned as an example of a conventional technique, a method of smoothing speckles by using “similarity” based on statistic in samples and statistically extracting a signal displaying non-Rayleigh scattering is proposed. In filtering using similarity in the method, a matrix of (M, N) pixels having a point P0 (x, y) as a center in ultrasonic receive RF signals disposed in a two-dimensional matrix obtained by sequentially disposing one-dimensional data of scanning lines is assumed, and a weighting factor as expressed by Equation 1 is computed with respect to all of points Pi in the matrix.wi={1−((Ii−I0)/ασ)2}2   Equation 1
Ii and I0 denote amplitude values at points Pi and P0, respectively, σ denotes a standard deviation in a sample, and α indicates an arbitrary filter factor. In the equation, when the inside of { } is negative, wi=0. By using wi obtained from Equation 1, an amplitude value at each point is multiplexed on the amplitude value at point P0 as shown by Equation 2.PoΣPiwi/Σwi   Equation 2
Although the filter is a smoothing filter in a broad sense, the filter is not related to distance between pixels. By Equation 2, the difference between amplitudes, that is, pixels having “similarity” are averaged.
FIGS. 1(A) and 1(B) are diagrams for explaining a problem to be solved by the invention and schematically illustrating reflection intensity of ultrasonic wave by continuous reflectors (structures).
FIG. 1(A) is a diagram schematically showing reflection intensity of ultrasonic waves by an interface between a structure—1(31) and a structure—2(32) which are continued in a living body. FIG. 1(B) is a diagram schematically showing reflection intensity of ultrasonic waves by an interface between a structure—3(33) and a structure—4(34) which are continued in a living body. 51 denotes a direction along the interface of the two structures, and 52 indicates a direction perpendicularly crossing the interface of the two structured.
FIG. 2 is a diagram for explaining a problem to be solved by the invention and schematically illustrating reflection intensity of ultrasonic waves by point reflectors 40, 41, 42, 43, and 44 which are scattered in a living body.
Reflection in a living body can be classified into the following two types (1) and (2).    (1) Reflection of ultrasonic waves by the interface (FIG. 1) of structures such as organs, blood vessel walls, tissues in an organ such as tumors, or thrombi in a blood vessel which are continued at least in one direction.    (2) Reflection by point reflectors (FIG. 2) which are not continued but are spread in a living body or the like.
In the following description, an image based on reflection intensity of the reflection (1) of ultrasonic wave, that is, an image (image in which a structure is reflected) obtained by emphasizing and extracting the structure of a living body tissue constructed by a set of point reflectors which are continuously distributed in at least one direction in the living body will be called a “structure-extracted image” or “structure-emphasized image”. An image based on the reflection intensity of the reflection (2) of ultrasonic waves, that is, an image (image in which texture of a tissue is reflected) obtained by extracting components resulting from a reflector constructed by a set of point reflectors which are not continuously distributed in a living body but are spread will be called a “texture-extracted image” or “texture-emphasized image”. A texture pattern resulting from properties of a tissue in a living body is one of living body information pieces and is utilized for diagnosis, as image information indicative of the properties of a tissue.
In the image acquisition using ultrasonic waves, in the case where the distribution of reflection intensity of ultrasonic waves from reflectors changes in a range almost equal to or smaller than width of point response function determined by the size of an aperture of transmit/receive wave, distance between the aperture and the reflector, and frequency of an ultrasonic pulse, echo signals from the reflectors interfere with each other, so that an interference pattern is multiplexed on an actual image in which the distribution of reflectors is reflected or the image is modulated by the interference pattern. It causes a problem such that the structure in the living body is not seen clearly.
As an attempt to make a structure in a living body clearly seen in an ultrasonic image, a method of removing the interference pattern has been examined. A normal linear filter has a drawback such that when efficiency of removing an interference pattern is increased, an edge of a structure is made blunt.
Generally, the spatial frequency in the distribution of reflectors in a structure is not always lower than that in the distribution of reflectors by which texture is obtained. The spatial frequency in the direction 52 perpendicularly crossing the interface of the two structures 33 and 34 shown in FIG. 1(B) is about the same as the spatial frequency of the distribution of reflectors from which texture is obtained. A change in intensity of an echo signal can occur at a high spatial frequency in the direction along the interface of the two structures 33 and 34. Therefore, although a B-mode image is adaptive emphasized by using two kinds of filters of a high-pass filter and a low-pass filter in the method described in Patent Document 1, it is difficult to satisfy both clear counter of a structure and removal of the interference pattern. It is improper to use a low-pass filter to eliminate an interference pattern from the viewpoint of picture quality. Regarding extraction of a structure in a living body and extraction of texture, the two kinds of filters do not correspond to the functions of an accelerator and a brake.
Although the filter described in the non-patent document 1 extracts a structure excellently, a problem occurs such that the filter erases a texture pattern. It is difficult to adjust the balance between extraction of a structure and extraction of a texture pattern which are mutually contradictory only by controlling the degree of extraction of a structure.